Reorder List

Problem

Given a singly linked list L: L0 → L1 → … → Ln-1 → Ln

reorder it to: L0 → Ln → L1 → Ln-1 → L2 → Ln-2 → …

Example

Given 1->2->3->4->null, reorder it to 1->4->2->3->null.

Can you do this in-place without altering the nodes' values?